Gasification processes are widely used to convert solid or liquid feedstocks such as coal, petroleum coke and petroleum residue into synthesis gas. Synthesis gas is predominantly composed of hydrogen gas (H2) and carbon monoxide (CO), and is utilized both as fuel for the production of electricity, as well as a feedstock for producing chemicals such as hydrogen, methanol, ammonia, synthetic natural gas or synthetic transportation oil.
Synthesis gas produced via the gasification of carbonaceous material commonly contains some methane. The relative quantity of methane in the synthesis gas varies with the type of gasification system utilized, but is often observed to be higher in two-stage gasification systems. For example, the synthesis gas produced in ConocoPhillips E-Gas™ two-stage gasifier usually contains between 1.5-4% methane (dry vol.) This quantity of methane produced is not of significant concern when the synthesis gas is utilized as fuel for gas combustion turbines that produce electricity. However, the presence of methane is not desirable when the synthesis gas is to be utilized as a feedstock for the production of value-added petrochemicals.
Known chemical production methods utilizing synthesis gas commonly involve converting H2 and CO from the raw synthesis gas into liquid products. Methane is often an inert gas for these syngas conversion processes. Thus, the purge gas leaving the chemical conversion reactor is often rich in methane. This purge gas is commonly combusted as a fuel gas, resulting in a reduction of the overall carbon conversion efficiency. Accordingly, there exists a need for improved processes and systems that can increase the overall carbon conversion efficiency of the gasification process, thereby maximizing the production of hydrogen and CO for a given quantity of feedstock. The invention described herein provides a unique process for improving the overall carbon conversion efficiency of the gasification process by recycling the methane-rich purge gas back to the gasification reactor where the methane-rich gas is converted to syngas via the steam-methane reforming reaction.